Vehicles are equipped with a variable-speed drive or transmission between the internal combustion engine and the driving wheel. The variable-speed drive changes the driving-wheel force and vehicle speed according to the vehicle traveling condition which often changes.
The variable-speed drive operates to change the belt ratio by changing the width of the groove of a pulley which includes a first pulley piece (disk) fixed to a rotating shaft and a second pulley piece (disk) installed on the rotating shaft so that it can be axially contacted with or separated from the first pulley piece through hydraulic pressure to change the engagement radius of the belt and the corresponding transfer of power.
The above type of continuously-variable-speed drive is disclosed in Japanese Publication No. 109661/85. The pulley piece or disk of the drive disclosed in the above publication has convexities and concavities of a size of 20 microns or larger formed on the disk surface to lengthen the service life by keeping the ratio of flat area to convex tip area at 20-70% and to improve the dimensional accuracy of the belt-driven continuously-variable-speed drive.
Another type of continuously-variable-speed drive is disclosed in Japanese Publication No. 184270/87. The drive disclosed in this publication is designed so that belts will intersect a V-groove of a block in which are formed several grooves with approximately the same width to adjust the friction between the block and pulley.
In the existing method of manufacturing the disk for a belt-driven continuously-variable-speed drive, the surface of the disks holding the metallic belt are made of blister steel and are precisely ground to finish the surfaces. However, when the tapered sheave surface of the disk is ground, there is the disadvantage that the equipment cost increases because a large and special grinder is needed to precisely maintain the surface configuration. Also, the grooves on the surface become shallow, and irregular swells are produced due to variations in the quality of the dressing of the grinding wheel.
Another disadvantage is that, because a friction drive system, unlike the so-called traction drive, is applied between the sheave surface of the disk and the belt, the existing manufacturing method can not be practically used since it is necessary to use a cooling oil which must then be removed to prevent the surface from being coated with an unnecessary oil film.
The purposes of the invention include providing a method of manufacturing economical disks for a belt-driven continuously-variable-speed drive capable of keeping the oil condition on the sheave surface of the pulley constant, generating compressed residual stress on the sheave surface, improving the fatigue strength of the disks, and decreasing the machining cost by forming a spiral groove with a surface roughness (i.e. height) of approximately 0.8-0.4 microns on the sheave surface of the disk by using a numerical control (NC) lathe and then grinding the sheave surface through super finishing in order to eliminate the above disadvantages.